The present invention relates to an electrolytic display cell with a metal deposit and more specifically to a counter-electrode construction for a cell of this type.
The presently known metal deposit electrolytic display cells generally comprise a transparent electrode of a form suitable for display purposes, a metal counter-electrode, an electrolyte placed between the said electrode and the said counter-electrode and means for supplying electric power to the electrode and counter-electrode in such a way as to cause the deposition or dissolving of a metal coating formed from the counter-eelectrode metal on certain parts of the electrode.
In liquid electrolyte cells of this type the counter-electrode conventionally comprises a more or less thick metal coating, for example a sliver coating deposited on an inert insulating support such as a sheet of glass and the display is obtained by bringing about on certain parts of the electrode the deposition of a coating of the metal constituting the counter-electrode, for example a silver coating, whose optical properties (reflection, absorption) will lead to the production of a symbol (character, curve, segment, etc.).
The counter-electrodes used in such cells have the disadvantage of suffering a certain deterioration during the operation of the cell, which leads to a deterioration of the appearance of the display and makes the cell virtually unusable after a limited time, generally corresponding to approximately 10.sup.5 switching cycles.
Thus, the operation of such a metal deposit electrolytic display cell is based on the electrochemical reaction: EQU M.revreaction.M.sup.n+ +ne.sup.-
in which M represents the metal of valence n which forms the metal deposit.
However, this reaction is not completely symmetrical, because the deposit corresponding to the reaction M.sup.n +ne.sup.- .fwdarw.M has an over voltage .eta..sub.c greater than the over voltage .eta..sub.a which characterises the reverse reaction M.fwdarw.M.sup.n+ +ne.sup.- corresponding to the dissolving of the deposit of metal M.
Further, as in an electrolytic display cell, the counter-electrode surface is very large compared with that of the active electrode which has a configuration suitable for display purposes and as a result during the display the counter-electrode metal is dissolved in a preferred manner on those parts of the counter-electrode facing those parts of the electrode used for display purposes whilst, during the erasure, metal M tends to be redeposited on a much larger surface of the counter-electrode, which in the long-term leads to a metal impoverishment or reduction in certain parts of the counter-electrode.
When the counter-electrode metal film is relatively thin, for example of the order of a few dozens microns, the impoverishment phenomenon can lead to the complete disappearance of the film in certain areas. In this case, during the display, the deposition of metal on the active electrode can only occur on the edges of the parts intended for display purposes, so that the appearance of the display becomes inhomogeneous and unacceptable and the cell is unusable.
To obviate this disadvantage consideration has been given to increasing the thickness of the metal coating constituting the counter-electrode, but this solution is relatively unsatisfactory because it requires a large quantity of metal, which raises in a by no means negligible manner the cost of the cell, particularly when the metal is silver.
Moreover an electrooptical display device with a solid electrolyte is known (French Pat. No. 2 260 167) in which a metal counter-electrode is partly covered with insulent as the electrode and thus has a shape complimentary to that of the electrode. In such a device, during erasure, the metal deposited on the electrode from the counter-electrode cannot be redeposited on a larger surface of the counter-electrode, so that there is no risk of an impoverishment of the metal in certain parts of the counter electrode.
However, this solution has the disadvantage of requiring, during the assembly of the cell, of the perfect coincidence between the motifs of the counter-electrode and the corresponding motifs of the electrode, which leads to certain difficulties.
French Pat. No. 1 435 521 also discloses a light control device constituted by an electrolyte inserted between a continuous electrode and a metal particle-generating counter-electrode which is constituted for example by a cadmium-covered platinum wire. In this device the application of a potential difference makes it possible to gradually deposit metal particles over the complete electrode surface. On erasure all these metal particles return to the electrode, so that there can be no impoverishment of the metal of the counter-electrode.